Shaping Of Gallate-based Metal-Organic Frameworks For Gas Adsorption Separation

Gallate-based metal-organic frameworks,termed as M-gallate(M=Mg,Co,Ni),have shown excellent performance for adsorption separation of C2 hydrocarbons(acetylene/ethylene,ethylene/acetylene).They are synthesized from gallic acid and metal salts,which are low-priced and environmentally friendly bulk chemicals,making M-gallate possess a prospect of industrial application.However,the usage of MOFs materials in powder from may face difficulties such as dustiness,pressure drop across fixed-bed,pipe blockage,quality reduction as well as challenges in their transportation and handling,all of which have hindered further industrial applications of MOFs.Shaping MOFs into application-oriented forms is an indispensable part of the industrialization of MOFs.In response to the challenges of the synthetic MOFs material forming process,we proposed a method using hydroxypropyl cellulose(HPC)as a binder to prepare a gallate-based MOFs molding material,which has a certain degree of mechanical performance while retaining excellent adsorption and separation performance.In addition,we applied this shaping method to other MOFs materials to provide basic data references for the molding and industrial applications of MOFs.We selected five binders:hydroxypropyl cellulose(HPC),hydroxyethyl cellulose(HEC),kaolin,silane coupling agent KH570,and polyvinyl butyral(PVB)to form Mg-gallate pellets with different binder content.We investigated the influence of shaping on physical properties,microstructure,adsorption performance and mechanical stability of MOFs.The results showed that the addition of five binders rarely affected the crystal structure and thermal stability of the MOF.Among these binders,the shaped particles with HPC had smooth surface and complete morphology.It can be observed that the binder forms small hook and cone-like structures among the dispersed crystals,and closely connected the crystal particles under the scanning electron microscope.Static adsorption experiment showed that the adsorption capacity of pellets shaped with HPC(content of 5%)only decreased by 1.42%.Even if the binder content was increased to 10%and 15%,the adsorption capacity decreased within 10%,indicating that HPC had almost no effect on the adsorption performance of Mg-gallate.In abrasion test,the pellets shaped with HPC showed no mass loss within 480 minutes.In compression experiment,the pellets shaped with HPC possessed the largest ultimate compressive strength(9.47 MPa),showing Excellent mechanical properties.We used HPC as binder to prepare pellets of M-gallate(M=Mg,Co,Ni).The single-component gas adsorption experiment of acetylene,ethylene,and ethane verified that the addition of HPC has little effect on the adsorption capacity of the material.The adsorption isotherm of the shaped pellets in the low-pressure zone showed a steep upward trend as same as powder materials,indicating that shaped pellets retianed strong interaction between the frameworks and the acetylene molecule.The IAST separation selectivity of three materials before and after shaping was calculated.The selectivity of the three materials to C2H4/C2H6(50:50,v/v)and C2H2/C2H4(1:99,v/v)mixed gas is slightly lower than that of powder materials at 298 K and 1 bar.But their separation performance was still better than most powder porous materials(such as 13X zeolite,5A zeolite and MgMOF-74).Breakthrough test was used to verify the dynamic separation performance of the shaped pellets.In the test of C2H4/C2H6 mixed gas,the breakthrough time of C2H6 was 10 minutes,whereas C2H4 was 80 minutes in the best-performing Co-gallate packed column.In the test of C2H2/C2H4,C2H4 breakthrough at 5 minutes,while the breakthrough time of C2H2 was 27 minutes.The significant difference in the breakthrough time of C2 hydrocarbons and five cycles of breakthrough tests proved that M-gallate pellets can effectively separate mixed gases in practical industrial applications and they had good recycling performance.We have applied this shaping process to other MOFs with different topological structures:CPL-n,MpzNi(M=Ni,Co),and squaric acid-based MOFs.It has been confirmed that the adsorption capacity of shaped CPL-n and MpzNi has only slightly decreased in single-component gas adsorption experiment.In the dynamic breakthrough experiment,excellent adsorption and separation performance was also maintained after shaping.However,the squaric acid-based MOFs materials losed its adsorption capacity to CO2 after shaping.The specific surface area of shaped pellets was almost zero.It is speculated that the specific surface area of the squaric acid-based MOF itself was low,the channels are one-dimensional,and the interaction between the adsorption sites in the framework and the adsorbate molecules is weak,which greatly increased the probability of the pores being blocked.